Ambipolar photoreceptor and method of imaging

ABSTRACT

AN AMBIPOLAR PHOTORECEPTOR MEMBER WHICH COMPRISES A SUBSTRATE AND A PHOTOCONDUCTIVE LAYER OVERLAPPING SAID SUBSTRATE. THE PHOTOCONDUCTIVE LAYER COMPRISES VITREOUS SELENIUM OR A SELENIUM-ARSENIC ALLOY WHICH IS DOPED WITH A SMALL AMOUNT OF SODIUM, LITHIUM, POTASSIUM, RUBIDIUM, OR CESIUM. THE PHOTORECEPTOR MEMBER EXHIBITS SATISFACTORY CHARGES ACCEPTANCE AND DISCHARGE FOR PHOTOGENERATED CHARGES OF EITHER POLARITY.

1972 J. J. GALEN 3,685,989

AMBIPOLOR PHOTORECEPTOR AND METHOD OF IMAGING Filed D66. 18, 1970 INVENTOR. JOSEPH J. GAL EN BY [Aw 1).

A T TORNE V 'U.S. Cl. 96-1 PC United States Patent 3,685,989 AMBIPOLAR PHOTORECEPTOR AND NETHOD 0F IMAGING Joseph J. Galen, Webster, N.Y., assignor to Xerox Corporation, Stamford, Conn. Filed Dec. 18, 1970, Ser. No. 99,430 Int. Cl. G03g 5/02, 13/22 6 Claims ABSTRACT OF THE DISCLOSURE An ambipolar photoreceptor member which comprises a substrate and a photoconductive layer overlaying said substrate. The photoconductive layer comprises vitreous selenium or a selenium-arsenic alloy which is doped with a small amount of sodium, lithium, potassium, rubidium, or cesium. The photoreceptor member exhibits satisfactory charge acceptance and discharge for photogenerated charges of either polarity.

BACKGROUND OF THE INVENTION This application relates to xerography, and more specifically, to a novel photoconductive device and a method of imaging.

In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the non-illuminated areas. This latent electrostatic image may then be developed to form a visible image by depositing finely-divided electroscopic marking particles on the surface of the photoconductive insulating layer. This concept was originally described by Carlson in U.S. Pat. 2,297,691, and is further amplified and described by many related patents in the field.

The use of vitreous or amorphous selenium, as described by Bixby in US. Pat. 2,970,906, remains the most widely used photoreceptor in commercial reusable zerography. Vitreous selenium is capable of holding and retaining an electrostatic charge for relatively long periods of time when not exposed to light, and is relatively sensitive to light as compared to most other photoconductive materials. In practice, vitreous selenium for use in xerographic plates is usually given a positive surface charge during the electrical sensitizing operation. This positive charging takes advantage of the better hole conduction through the selenium layer during illumination, in that selenium has a much more efficient discharge for hole than electron conduction. Therefore, unless specially treated, or other special precautions are taken, selenium generally has a suitable long range for holes and in general must be positively charged.

It should be pointed out that vitreous selenium does conduct both electrons and holes, but that the mobility of holes is approximately ten times greater than that for electrons. Thus, it can be stated that vitreous selenium i which it is sensitive, usually in the blue-green portion of the visible spectrum. The absorption of activating radiation acts to create hole-electron pairs in the selenium at the point of absorption of the impinging radiation. If the sensitizing charge on the surface of the selenium is negative, positive charge created by the radiation remains at the surface to neutralize existing negative charges while the photogenerated negative charges are repelled by the remaining sensitizing charge to migrate through the selenium toward the conductive backing. When the sensitizing charge on the surface of the selenium is positive, the reverse is true. Electrons created by the radiation remain at the surface to neutralize positive charges and the photogenerated holes or positive charge carriers are repelled to migrate through the selenium to the conductive backing. Inasmuch as selenium has a very short range for electrons, when used with negative charging, the result is that a large number of electrons are trapped in the bulk of the selenium layer, thereby rendering the plate unfit for further use in xerography until the trapped charges are freed. In that selenium has a long range for holes, when used with positive sensitizing charges, trapping is reduced to a sufficiently small degree so as not to interfere with the utility of the material for xerographic processes. It therefore has become the usual practice in xerography, when using vitreous selenium, to employ positive polarity sensitizing charges at its surface.

There are, however, some applications in xerography where in a short range for a minority charge carrier constitutes a critical deficiency. For example in xeroradiography, X-rays are used which penetrate the entire film of selenium, generating electron-hole pairs throughout the photoconductive layer, rather than merely near the surface as in the case of normal xerographic imaging. In order to absorb as large a portion of the X-ray energy as possible, xeroradiography requires the use of selenium films considerably thicker than those used in normal xerography. A convention xerographic plate is one designed for use with visible radiation, and is generally about 20 to microns in thickness. In xeroradiography, however, the selenium films are from about to 200 microns thick. The result of this combination of circumstances is that to be useful in xeroradiography, the photoconductive insulating material must have an appreciable range for both holes and electrons.

Another example where it is desirable that vitreous selenium have a long range for both polarities of charge carriers is in obtaining a reversal of the image to be re produced in the normal xerographic process. In this case, if the normal xerographic plate is charged negatively and then the steps of the xerographic process carried through, including development with carriers and toners as described for the normal xerographic process, there is obtained a negative or reversal image of the copy being reproduced. Thus, if the plate has a long range for both polarities of charge carriers it is possible merely by altering the polarity of the sensitizing charges to obtain either a positive or reversal reproduction of the subject matter being reproduced.

US. Pat. 3,077,386 to Blakney et al. describes one technique for treating selenium whereby the material acquires the property of having a long range for both polarities of charge carriers. This technique involves doping the selenium with a small amount of metal such as chromium, nickel, iron, zinc, calcium, titanium, or other similar material.

There has now been discovered, an alternative method of rendering selenium and selenium-arsenic alloys suitable for use in methods where a satisfactory carrier range for both polarities of charge carriers is required.

3 OBJECTS OF THE INVENTION It is, therefore, an object of this invention to provide a novel device, containing a selenium or selenium alloy photoconductive layer which exhibits ambipolar characteristics.

It is another object of this invention to provide a novel photosensitive device which exhibits ambipolar characteristics.

It is a further object of this invention to provide a system utilizing a photoconductive layer which exhibits satisfactory charge acceptance and discharge for charges of both positive and negative polarity.

SUMMARY OF THE INVENTION The foregoing objects and others are accomplished in accordance with this invention by providing a xerographic member which provides satisfactory charge acceptance and discharge for charges of both polarities, and which may be referred to as an ambipolar photoreceptor. The instant concept is based upon the use of a photoreceptor member which utilizes a hole trapping photoreceptor layer which comprises vitreous selenium or a seleniumarsenic alloy which contains a small amount of sodium, lithium, potassium, rubidium, cesium, or mixtures thereof, in a concentration of about 5 to 5000 parts per million by weight. One embodiment of this invention comprises a conducting substrate overlayed with a photoconductive layer comprising vitreous selenium doped with sodium in a concentration of about 5 to 5000 parts per million by weight of the photoconductive layer.

BRIEF DESCRIPTION OF THE DRAWING The advantages of the instant invention will become apparent upon consideration of the following disclosure of the invention, especially when taken in conjunction with the accompanying drawing, wherein:

The figure represents a schematic illustration of one embodiment of a xerographic member as contemplated for use in the instant invention.

DETAILED DESCRIPTION OF THE DRAWING In the drawing, reference character illustrates one embodiment of an improved photoreceptor device of the instant invention. Reference character 11 designates a support member which is preferably an electrically conductive material. The support may comprise a conventional metal such as brass, aluminum, steel, or the like. The support may also be of any convenient thickness, rigid or flexible, and in any suitable form such as a sheet, web, cylinder, or the like. The support may also comprise other material such as metallized paper, plastic sheets covered with a thin coating of aluminum or copper ioide, or glass coated with a thin conductive layer of chromium or tin oxide. An important consideration is that the support member be somewhat electrically conductive, or have a conductive surface or coating, and that it be strong enough to withstand a certain amount of handling. Reference character 12 designates a photoconductive layer which may comprise vitreous selenium or a selenium-arsenic alloy containing a small, but critical amount of sodium, lithium, potassium, rubidium, cesium or mixtures thereof in the range of about 5 to 5000 parts per million by weight of the photoconductive layer.

The thickness of layer 12 is not particularly critical and may range from about 10 to 200 microns. In general, thicknesses in the range of about to 80 microns are particularly satisfactory.

The photoreceptor layer of the instant invention may be prepared by any suitable technique. A preferred technique includes vacuum evaporation where the photoconductive layer is evaporated onto its corresponding base material under vacuum conditions varying from about 10 to 10' Torr.

Another typical method includes flash evaporation in 4 which a powdered alloy such as sodium doped vitreous selenium is selectively dropped into a heated crucible maintained at a temperature of about 300 to 600 C. The vapors formed by the heated alloy are evaporated upward onto a substrate supported above the crucible.

In all of the above methods, the substrate onto which the photoconductive material is evaporated is maintained at a temperature of about 50 to C. If desired, a water cooled platen or other suitable cooling means may be used in order to maintain a constant substrate temperature. In general, a selenium layer thickness of about 60 microns is obtained when vacuum evaporation is continued for about 1 hour at a vacuum of about 10* Torr. at a crucible temperature of about 280 C. U.S. Pats. 2,803,542 to Ullrich; 2,822,300 to Mayer et al.; 2,901,348 to Dessauer et al.; 2,963,376 to Schaffert; and 2,970,906 to Bixby all illustrate vacuum evaporation techniques which are suitable in the formation of alloy layers of the instant invention. The crucibles which are used for the evaporation of photoreceptor layers may be of any inert material such as quartz, molybdenum, stainless steel coated with a layer of silicon monoxide, or any other suitable equivalent material. In general, the selenium or selenium alloy being evaporated is maintained at a temperature of above about its melting point.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples further specifically define the present invention with respect to a method of making and imaging a photoreceptor suitable for use with either positive or negative charging.

EXAMPLE I A xerographic drum made according to the instant invention is prepared as follows. Two quartz evaporation boats are placed in a vacuum evaporation chamber. Each boat contains grams of xerographic grade vitreous selenium containing parts per million by weight of sodium. The selenium is xerographic grade having a purity of about 99.999 percent and available from Canadian Copper Refiners. The sodium doped selenium is in the form of pellets ranging in diameter from about A; to A inch in diameter. The boats are connected directly to a source of electrical power adaptable to control their temperature. A substrate in the form of an aluminum drum blank 3 inches in diameter and 10 inches long having an aluminum oxide interface is suspended about 11 inches above the evaporation boats. The chamber is evacuated to a vacuum of about l0 Torr. and the boats heated to a temperature of about 285 C. for about 31 minutes to form a 57 micron layer of sodium doped vitreous selenium on the drum surface. At the end of this time, the vacuum chamber is cooled to room temperature, the vacuum broken, and the photoconductive drum plate removed from the chamber.

EXAMPLE II A control drum consisting of a single 60 micron layer of undoped xerographic grade vitreous selenium is prepared on a brass substrate by the method of Example I. The boats are heated to about 285 C. for about 45 minutes to form the 60 micron vitreous selenium layer on the drum surface. Both drums are then tested to measure their charge acceptance for both negative and positive polarity.

The drum made according to Example I, which illustrates one embodiment of the instant invention, accepts up to about 2000 volts of positive charge and is suitable for xerographic imaging in the conventional xerographic manner. This drum also accepts up to about 1000 volts of negative charge and also is capable of being used in the conventional xerographic manner using negative charging. The drum prepared by Example II, which does not utilize sodium doping, accepts 2700 volts of positive potential on positive charging. However, upon negative charging, this drum, which does not contain sodium, accepts only half the potential (about 500 volts) as the plate of Example I.

The drum made according to Example I is capable of being cycled in the conventional xerographic manner which includes charging to a negative potential, exposure to a pattern of activating radiation, followed by development with toner particles to form a visible image. The toner image is normally transferred to a sheet of paper. In order to keep the residual potential at a controllable level, the drum is uniformly exposeed to a precharge of positive corona before repeating the imaging cycle.

Although specific components and proportions have been stated in the above description of the preferred embodiments of this invention, other suitable materials and procedures such as those listed above, may be used with similar results. In addition, other materials and changes may be utilized which synergize, enhance, or otherwise modify the photoreceptor and method of use.

Other modifications and ramifications of the present invention would appear to those skilled in the art upon reading this disclosure. These are also intended to be within the scope of this invention.

What is claimed is: l

1. An ambipolar photoreceptor member which comprises a substrate and a photoconductive layer overlaying said substrate, said photoconductive layer consisting essentially of vitreous selenium or a selenium-arsenic alloy which is doped with a material selected from the group consisting of sodium, lithium, potassium, rubidium, cesium, and mixtures thereof in a concentration of 5- 5000 parts per million by weight of the photoconductive layer.

2. The member of claim 1 in which the photoconductive layer is doped with sodium.

3. The member of claim 1 in which the thickness of the photoconductive layer is from about 20 to microns.

4. A method of imaging which comprises:

(a) providing an ambipolar photoreceptor member which comprises a substrate and a photoconductive layer overlaying said substrate, said photoconductive layer consisting essentially of vitreous selenium or a selenium-arsenic alloy which is doped with a material selected from the group consisting of sodium, lithium, potassium, rubidium, cesium and mixtures thereof in a concentration of 5-5000 parts per million by weight of the photoconductive layer;

(b) uniformly electrostatically charging said member to a negative potential; and

c) exposing said charged surface to a pattern of activating electromagnetic radiation to form a latent electrostatic image.

5. The method of claim 4 in which the latent electrostatic image is developed to form a visible image.

6. The method of claim 4 in which the photoconductive layer is doped with sodium.

References Cited UNITED STATES PATENTS 2,803,542 8/1957 Ullrich 96-15 3,041,166 6/1962 Bardeen 96-1.5 3,483,028 12/1969 Bell et a1 252501 X 3,077,386 2/1963 Blakney et a1. 961.5 X 3,489,560 1/1970 Joseph 961.5

CHARLES E. VAN HORN, Primary Examiner U.S. Cl. 'X.R. 96-1.5; 252501 

